In the production of bevel and hypoid gears, two types of processes are commonly employed, generating processes and non-generating processes.
In generating processes a rotating tool is fed into the workpiece to a predetermined depth. Once this depth is reached, the tool and workpiece are then rolled together in a predetermined relative rolling motion, known as the generating roll, about the axis of a theoretical generating gear as though the workpiece were rotating in mesh with the theoretical generating gear, the teeth of the theoretical generating gear being represented by the stock removing surfaces of the tool. The profile shape of the tooth is formed by relative motion of the tool and workpiece during the generating roll.
Generating processes can be divided into two categories, face milling and face hobbing. In generating face milling, each slot (adjacent tooth sides of consecutive teeth) of a workpiece is formed individually. After the tool has been fed to its predetermined depth, the generating roll is commenced.
In the generating roll, tooth surfaces may be formed by any of several known cycles. One tooth surface of a slot may formed by a forward generating roll and the adjacent tooth surface formed by a generating roll in the reverse direction. Alternatively, both sides of the tooth slot may be cut in a single forward generating roll and if a secondary or finishing cut is required, this may be accomplished by a reverse roll to produce the desired tooth surfaces. Once the tooth sides are completed, the tool is withdrawn relative to the workpiece and the workpiece is then indexed to the next slot position. This intermittent indexing is continued until all tooth surfaces on the workpiece are formed.
Generating face hobbing is a continuous indexing process wherein a predetermined timed rotation between the tool and workpiece is superimposed on the generating roll. In this manner, all slots (and therefore all tooth surfaces) on the workpiece are formed by a single generating roll. The tool and workpiece are rotated in a timed relationship and the tool is then fed into the workpiece thus removing stock from all slots as it is fed to depth. Once full depth is reached, the desired generating cycle is commenced to completely form all teeth on the workpiece.
Non-generating processes, either intermittent indexing or continuous indexing, are those in which the profile shape of a tooth on a workpiece is produced directly from the profile shape on the tool. The tool is fed into the workpiece and the profile shape on the tool is imparted to the workpiece. While no generating roll is employed, the concept of a theoretical generating gear known as a "crown gear" is applicable in non-generating processes. The crown gear is that theoretical gear whose tooth surfaces are complementary with the tooth surfaces of the workpiece. Therefore, the cutting blades on the tool represent the teeth of the crown gear when forming the tooth surfaces on the non-generated workpiece.
In non-generating and generating processes, the first step is to engage the cutting or grinding tool with the workpiece and then feed the tool to the predetermined depth in the workpiece. The tool may be moved toward the workpiece, the workpiece may be moved toward the tool, or, both the tool and workpiece may be moved toward one another. Regardless of which of the tool and/or workpiece is moved, the tool must reach its predetermined plunge depth in the workpiece before the stock removal is complete and the tool withdrawn (non-generating processes) or the generating roll is commenced (generating processes).
The path along which the tool travels from its initial contact with the workpiece to its predetermined or "full" depth may vary. As discussed by Goldrich in "CNC Generation Of Spiral Bevel and Hypoid Gears: Theory and Practice", The Gleason Works, Rochester, N.Y., 1990, in conventional mechanical gear cutting and/or grinding machines comprising a cradle, which emulates the theoretical generating gear, the feedpath is in a direction along the cradle (generating gear) axis. While this type of tool feeding is well established, uneven tool loads or stresses have been noted.
The same Goldrich publication also discloses a CNC multi-axis gear generating machine wherein the feedpath of a tool into a workpiece may be in a direction along the tool axis. In feeding along the tool axis, the feedpath lies in a plane defined by the tool axis and a tooth normal vector at the design point (the point around which the geometry of the tooth is calculated). This plane is referred to as the "normal plane". The design point is usually the mean point of the tooth, the mean point being a point midway root-to-top and toe-to-heel on the tooth. This type of feeding is advantageous in cases where tool entrapment may occur when the feedpath is along the cradle axis. Tool entrapment is the condition whereby the tool removes excess stock material due to its particular presentation and motion relative to the workpiece. Feeding along the tool axis presents the tool to the workpiece in a manner that tool entrapment is prevented. However, feeding along the tool axis has exhibited excessive uneven tool loads especially in generating face hobbing processes.
Goldrich further discloses that feeding of the tool may take place at an angle relative to the tool axis in order to balance chip loads for cutting blades having unequal pressure angles. When cutting tools having unequal pressure angles are utilized, one side of the blades may remove more stock material than the other side thus causing one set of blades to wear faster than the other set of blades. When feeding a tool along the tool axis, the condition of uneven tool loads is further aggravated by utilizing tools having such unequal pressure angles. In this situation, the feedpath may be changed to an angle relative to the tool axis, but, the feedpath remains within the normal plane. This angled feedpath within the normal plane allows the tool to contact the workpiece in a manner such that stock removal between the two sides of the tool is balanced and blade wear due to unequal stock removal is alleviated. However, it should be noted that while feeding in the normal plane at an angle relative to the tool axis may reduce the wear and vibrational problems associated with unequal tool pressure angles, this type of tool feeding has not been found to remedy the excessive and uneven tool loads associated with feeding along the tool axis especially in generating face hobbing processes as noted above.
It is an object of the present invention to provide a method of feeding a tool relative to a workpiece wherein uneven tool loads associated with prior art feedpaths are reduced or eliminated.